I want to bring in co-an or the of "good morning america," an or the of "this week," george stephanopoulos. So, george, can a vice president ever turn the course of a campaign in a debate? Reporter: It's never happened before, diane, but tonight could be the night because of the timing. Coming obvious that big win by romney in the debate last week, paul ryan, if he can get a second clear win for the republicans, could transform this race. It's been tightened so far, hasn't been completely transformed. A big win tonight could do that. So, tell us what to watch for. What's the a-game? Reporter: The a-game for paul ryan is two things. He's got to make a human connection tonight. The other big thing he wants to do is try to force a mistake out of biddeen. He's made them in the past, but The big goal for biden is to show democrats out in the country, this ticket still has fight in them. Wh that's what he's got to do.

This transcript has been automatically generated and may not be 100% accurate.

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